Heat Exchangers used for two phase refrigerant evaporation for air cooling and/or dehumidification of air or gases, such as with heating, ventilation, air conditioning and refrigeration (HVAC&R) systems have historically encountered formidable challenges, requiring customized designs to be configured to operate properly, while achieving acceptable thermal performance while preventing adverse operating conditions such as oil logging, unstable operation, part load operation inefficiencies, liquid pass-through that damages compressors, and other undesirable conditions. In a known heat exchanger 10 having traditional fin and tube evaporator coils or tubes, as shown in FIG. 1, a refrigerant distributor 12 with feeder tubes 14 is used to provide refrigerant into individual or groups of tubes 16 in the coil. Refrigerant velocities, size, and/or enhancement of tubes 16, overall pressure drop in tubes 16, in combination with distributor 12 comprised of feeder tubes 14 are provided in an attempt to achieve equal or sufficient refrigerant distribution into heat exchanger 10, prevent oil drop out or oil logging, prevent refrigerant logging and surging, despite operating in adverse operating conditions. A control valve (not shown), controls the amount of refrigerant injected into heat exchanger 10 based on evaporator temperature, pressure and/or superheated refrigerant 20 exiting heat exchanger 10 via an outlet 22 of a refrigerant outlet header 24.
A stacked, brazed plate heat exchanger 26, typically used as a refrigerant evaporator for fluid cooling is generally depicted in FIGS. 2 and 3. Embossed plates 28 are stacked, with adjacent plates defining a fluid channel for flow of refrigerant 20 such that every other fluid channel between a refrigerant inlet 34 and a refrigerant outlet 36 becomes a refrigerant channel for cooling a fluid 30 flowing through a corresponding fluid channel between a fluid inlet 38 and a fluid outlet 40. A refrigerant distribution tube or distributor tube 32 is then inserted into refrigerant inlet 34. Distributor tube 32 has orifices positioned along a lower portion of distributor tube 32 and pointed downward in a direction substantially opposite a primary flow direction 44 (FIGS. 2 and 4) of refrigerant 20 such that refrigerant 20 is discharged from refrigerant distributor tube 32 from orifices 42 in an initial flow direction 46 prior to turning and flowing in primary flow direction 44. This distributor tube construction for brazed plate heat exchangers has been sold in the United States since the early 1990's.
FIG. 4 is based from an actual photograph showing a cross section taken along line 4-4 of FIG. 3 of the lower section of plate heat exchanger 26, showing refrigerant inlet 34 and fluid outlet 40. Shown together are refrigerant inlet 34, distributor tube 32 with 0.08 inch (2 mm) orifices 42, and plate channels 48. When operating, refrigerant 20 enters refrigerant inlet 34 and proceeds interior of distributor tube 32, the refrigerant flow being metered or controlled through orifices 42 and entering heat exchanger channels 48 formed between alternating adjacent plates 28. Upon entering the heat exchanger channels 48, the initial refrigerant flow direction 46 (FIG. 2) is turned in a direction substantially primary opposite flow direction 44 to flow into plate channels 48 along a heat transfer surface 39 toward refrigerant outlet 36 (FIG. 2). FIG. 4 shows a gap 50 between plate port opening 52 and outer diameter 54 of distributor tube 32. In a later version, outer diameter 54 of distributor tube 32 tightly fits inside plate port opening 52. Orifices 42 are typically positioned at a 6 o'clock or 5 o'clock orientation relative to the direction of primary refrigerant flow direction 44 (12 o'clock orientation).
Other innovations in brazed plates included recessed features punched into the plates or plate ports. Another innovation used a tube of sintered metal which, when inserted into the refrigerant inlet of the plate stack, provided atomization, with limited success. While heat exchanger arrangements utilizing tubes have improved refrigerant distribution, multiple challenges remain. These challenges include oil drop-out at full and part load, inconsistent or below expected performance at part load, operational stability, and limitations associated with refrigerant injection, which limits the number of plates or depth that can be effectively used in a plate heat exchanger.
The development of flat tubes with ultra small multiport openings, also called Microchannel tubes, as are known in the art, when configured as a heat exchanger evaporator used for cooling air (gas) in an air cooling or dehumidifying system, offering opportunities for improved operational efficiencies. However, complexities and issues involving refrigerant distribution and optimal coil performance are many and need to be resolved. These complex issues and phenomenon include, but are not limited to:                effects of entrance velocity of the refrigerant to be cooled;        liquid to gas ratio at inlet;        orifice pressure drop along the inlet manifold;        vertical re-direction of refrigerant upward to the multiport tubes;        lateral re-direction of refrigerant flow to a large number of multiple parallel tubes;        refrigerant liquid dropout and liquid/gas recombination;        liquid/gas separation;        vertical flow and effects of gravity;        effects of manifold header length or depth;        secondary mal-distribution of refrigerant into the multiport tubes,        compressor oil drop out;        oil pass-through and pooling;        minimum refrigerant velocities;        outlet header dynamics and pressure drop;        refrigeration system operation from 100% capacity to 10% capacity;        minimal refrigerant charge requirements; and        consideration of refrigerant type characteristics, such as R410a (high pressure, low volumetric gas) versus R134a (low pressure, high volumetric gas).        
U.S. Pat. No. 7,143,605 is directed to improve refrigerant distribution for Microchannel tubular heat exchangers. Although U.S. Pat. No. 7,143,605 utilizes previously known prior art and geometries similar to the tubular distributor used in brazed plate heat exchangers previously described, this patent also suffers from several technical deficiencies and omissions. In actual practice and observation, these deficiencies are confirmed in brazed plate heat exchangers and confirmed in Microchannel tubular heat exchangers as identified below.
Other methods attempted for use with heat exchangers having tubes or plates, such as U.S. Pat. No. 6,688,137, relate to direct feed tube injection into the headers and refrigerant recirculation. Such methods all have tried to induce and improve the distribution feed of the entering liquid and gas combination of refrigerant, but most solutions have limited functionality or range of operation, or single design point operation.
Through visual observation, testing, and desired design attributes for an air to evaporating refrigerant heat exchanger, an improved refrigerant distributor of such a heat exchanger is disclosed herein to incorporate novel features and functionality required to efficiently work for Microchannel tubular heat exchangers. The heat exchanger of the present disclosure works in combination with vertical tube orientation and, to work in combination with normal and over-sized manifold headers for optimum thermal performance, and, to counteract the effects of outlet header manifold pressure drop and, to provide uniform refrigerant distribution in the inlet manifold and, to provide uniform injection across all the multiport tubes, over a wide range of operating conditions and design issues. In addition, the heat exchanger of this disclosure will work at any Microchannel tube or refrigerant tube orientation between vertical and horizontal as an evaporator or condenser.
The distributor of the present disclosure can also be operated in reverse refrigerant flow for heating duty in a refrigerant heat pump system, and by using standard automatic switching valves that allow the same evaporator heat exchanger to then be used as a condenser for heating operation.
In addition, the distributor of the present disclosure can be applied to historical Microchannel heat exchanger configurations with round header manifolds (FIGS. 18-21) and non-round header manifolds.
The operation of the heat exchanger of the present disclosure differs from the brazed plate type heat exchanger. In the brazed plate heat exchanger, the refrigerant, after passing through the distributor ports, directly enters the heat transfer surface which promotes refrigerant boiling, creation of gas to propel the refrigerant upward into the plate structure. Whereas, in one embodiment of the heat exchanger of the present disclosure, the refrigerant must pass through the distributor orifices, be directed to the tube area, where each tube is isolated from the adjoining tube, and, the refrigerant is then injected into the tube entrance areas, and where a second refrigerant distribution characteristic is accommodated.
The heat exchanger of the present disclosure differs significantly from U.S. Pat. No. 7,143,605 and the other known art in many ways, including features achieving a deliberate gas/liquid separation of fluid delivered to the distributor, use of a weir arrangement to facilitate refrigerant liquid injection into orifices formed in the distributor, directional control of the refrigerant flow to the inlet or inlet header and then to the Microchannel or multiport tubes or refrigerant tubes, use of secondary openings to create a pressure drop to propel the refrigerant and to spread out the liquid substantially evenly across the length of the header, a ternary set of openings to inject refrigerant into the tube chamber(s), isolation of each tube as mini-chambers or secondary chambers to prevent refrigerant flow between refrigerant tubes prior to entering the tubes, the use of a surface geometry or surface features for holding and capturing refrigerant liquid so as to feed the multiport tube(s) or refrigerant tubes, and method of modifying the tube entrance to alter the refrigerant distribution into the multiport tube or refrigerant tube.